1. Field of the Invention
This invention relates to devices for detecting the presence of motor-driven objects, and in its preferred embodiment relates to a system for detecting the presence of motor vehicles for use in controlling vehicle traffic.
2. Description of the Prior Art
In the past, a variety of vehicle detectors have been used to monitor the flow of traffic and control traffic signals in response to the traffic flow. One type of vehicle detector is the so-called "pulse detector" which senses the passage of a vehicle moving past a sensor and generates a pulse for each vehicle sensed. The pulse is fed to a suitable control circuit for either counting the vehicles or activating a traffic signal controller, for example. The main disadvantage of pulse detectors is that a vehicle must be in motion before it can be sensed by the detector. A vehicle stopping in the vicinity of the sensor is not sensed continuously while it is in the presence of the sensor. Thus, when the vehicle departs from the vicinity of the sensor, no instruction is provided by the sensor to the control circuit to indicate that the vehicle is no longer present. Accordingly, pulse-type detectors are not particularly suitable for traffic signal control where it is desired to continuously know whether a vehicle is present at a signal. For example, pulse detectors will instruct a traffic signal controller that a certain number of vehicles have passed a certain detection point. However, since they do not monitor the continuous presence of vehicles which may be standing idle waiting for a signal to change, they are unable to instruct a signal controller to change the signal once all the vehicles have left the detection point.
Another major type of vehicle detector is the so-called "presence detector" which senses vehicular traffic whether or not the vehicles are in motion and is able to provide a continuous signal to a control circuit as long as the vehicle being sensed in in the vicinity of the presence detector. The most common type of vehicle presence detector used today is the so-called "inductive loop" detector in which a large inductive loop formed by a coil comprising two or three turns of a relatively large diameter conductor wire (preferably No. 12 wire) is embedded in a traffic lane. The inductive loop is part of a tuned circuit for controlling the frequency of an oscillator. A vehicle entering the field of the loop will vary the inductance value of the loop and thereby change the loop circuit resonant frequency characteristics. This is sensed to provide an output signal which is fed to a traffic controller for controlling a set of traffic signals.
There are several disadvantages of the inductive loop vehicle-presence detectors. For example, inductive loop detectors basically are "metal detectors," since the presence of a relatively large metallic object in the magnetic field of the inductive loop is necessary to detune the loop circuit to indicate the presence of the object. Relatively large metal masses such as automobiles, trucks, and buses are easily detected magnetically, but unfortunately smaller objects such as motorcycles often are not detected. Moreover, extremely large metallic objects, such as a bus or a large truck passing in a given lane of traffic may be detected by sensors located in two adjacent traffic lanes and therefore counted as two passing vehicles rather than one. Large stationary metallic masses, such as a nearby parked automobile, often will be continuously sensed by the inductive loop which then locks up the operation of the detector circuit, and thereby interferes with the proper operation of the traffic signal controller.
The detector circuit of inductive loop detectors generally operates in the RF (radio frequency) band in the range of about 100 KC. Operating on radio frequencies limits the effective cable length between the inductive loop pickup and the output amplifier because of the combined added capacitance, resistance, and inductance of the electrical line carrying signals at these frequencies. Besides the sensitivity to cable length, radio frequency operation also makes the inductive loop detectors sensitive to environmental changes and cross-talk between adjacent pickup loops and lead-in wires.
The typical inductive loop is about a 6-foot square coil which requires removal of a substantial amount of pavement to embed the loop in a traffic lane. This results in a relatively high installation cost for the unit. The initial cost of inductive loop detector systems also is relatively high because of the use of rather sophisticated tuned oscillator circuits and related circuitry in the detector.